1. Field of the Invention
The present invention is generally related to electronic devices, and is more specifically related to circuits for reducing electromagnetic interference (EMI) generated by electric motors.
2. Description of the Related Art
Electromagnetic interference (EMI) is a disturbance that may interrupt, obstruct, degrade or limit the effective performance of an electronic circuit or electronic device. An EMI disturbance may be due to either electromagnetic conduction or electromagnetic radiation emitted from a source, such as an electric motor. Electromagnetic conduction and electromagnetic radiation are differentiated by the way the electromagnetic field propagates. Conducted EMI is caused by physical contact between conductors, and radiated EMI is caused by induction, without physical contact between conductors.
An EMI disturbance may result in adverse consequences including the uneven distribution of an electromagnetic field around a conductor, skin effects, proximity effects, hysteresis losses, transients, voltage drops, electromagnetic disturbances, EMP/HEMP, eddy current losses, harmonic distortion, and reduction in the permeability of a material.
One solution for minimizing a device's susceptibility to EMI is electromagnetic shielding. EMI shielding, however, is expensive and has negative consequences. Another method to reduce EMI involves twisting wires, however, many facilities have tens of thousands of feet of wire, so this solution is not always practical.
Another solution for minimizing EMI emissions includes providing an electronic device with an EMI filter. For example, U.S. Pat. No. 6,400,058 to Liau discloses a universal motor having reduced EMI emission characteristics. The universal motor has a stator, a rotor, and brushes. The EMI filter includes a filter circuit provided on a printed circuit board (PCB). The PCB including the filter circuit is mounted over the brushes on the motor. When electrical arcing occurs between the commutator and the brushes, the PCB filter acts as a shield that absorbs a portion of the radiation emitted by the arcing so as to reduce the EMI characteristics of the universal motor.
Most countries have legal requirements that mandate the electromagnetic compatibility of devices that produce electromagnetic fields. These legal requirements mandate that equipment manufacturers produce electronic devices that work properly when subjected to certain levels of EMI, and that do not emit EMI at levels that will interfere with other equipment.
In 1982, the United States enacted Public Law 97-259, which granted the Federal Communications Commission (FCC) the authority to regulate consumer electronic equipment for EMI. After the law was promulgated, the FCC worked with equipment manufacturers to develop acceptable EMI standards for electronic hardware.
Under the FCC compliance program, electronic devices must be tested to insure that they meet acceptable EMI standards. There are generally three types of EMI compliance tests: emission testing, immunity testing, and safety testing. Emission testing insures that a product will not emit harmful interference by electromagnetic radiation. In one emission test, one or more antennas are used to measure the amplitude of the electromagnetic waves emitted by a device. The amplitude of the emitted waves must be under a set limit, with the limit depending upon the classification of the device. Immunity testing insures that a product is immune to common electrical signals and EMI disturbances that will be found in its operating environment, such as electromagnetic radiation from a local radio station or interference from nearby products. Safety testing insures that a product will not create a safety risk from situations such as a failed or shorted power supply, and power line voltage spikes and dips.
One type of EMI testing device is sold under the trademark R&S® ESU by Rohde & Schwarz GmbH & Co. The R&S® ESU device combines a testing receiver and a spectrum analyzer in one component. The R&S® ESU device makes a wide variety of measurements related to EMI testing including peak, AV, RMS, CISPR-AV and quasi-peak measurements.
In spite of the above advances, there remains a need for improved, reliable, and economical EMI reducing circuits for electronic devices that minimize the likelihood of EMI disturbances and that meet or exceed FCC standards.